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Failure phenomenon: the left rear turn light is not bright. Inspection and analysis: first test the car to verify the failure phenomenon. When the steering light is switched on, the steering light at the right front, right back and right wing can be lit normally; when the turning light switch is switched on the left turn light, the steering light at the left wing and the left wing can be lit normally, but the left rear turn light is not bright. Connect the Ford special tester IDS to read the trouble code. There is no fault code store. First, check the left rear steering bulb according to the principle of simplicity to complexity. Remove the left rear taillight and check the steering bulb. The bulb is good. Use the test lamp to measure the turn off bulb end connector terminal and turn on the left turn signal. The test light is not lit. The car's left rear steering lamp is supplied by the body control module (BCM), and the car's left rear steering lamp is supplied by the car body control module (BCM). Because the rear brake light, the profile lamp and the reversing lamp are normal, it is possible to exclude the possibility that the iron wire is bad for the left rear turn light, so the fault point is locked on the power supply line of the left rear turn lamp. Remove the left front A pillar lower trim panel and measure the F14 power supply fuse of the left rear steering lamp. Next, a multimeter is used to measure the conduction of the power line between the BCM and the left rear turn lamps. Thus it is determined that the fault is caused by BCM damage. Exclusion method: replace BCM.